1. Field of the Invention
The present invention relates to communications within a private telephone system, particularly, to communications between a multiplicity of users via a private base station, and, more particularly, to an improved system and method for intercom communications between users within the private telephone system and for multiple voice communications between users within the private telephone system and remote users outside the private telephone system.
2. Background and Objects of the Invention
The past two decades have seen a considerable rise in the deployment of mobile telephony across the globe. As noted in U.S. Pat. No. 5,555,258, however, mobile telephony was preceded by cordless telephony, a low-power, low-range ancestor used primarily in the residential context and enabling an individual to move around a house or apartment and still place and receive calls. As shown in FIG. 1, a conventional cordless telephone system, designated generally by the reference numeral 10, includes a private or home base station 12 and a plurality of portable or cordless phones 14, e.g., cordless phones 14A and 14B, which are coupled to the private base station 12 through radio frequency (rf) signals. The private base station 12 is hardwired to a Public Switched Telephone Network (PSTN) 16, whereby a cordless phone user, e.g., at phone 14A, may access and communicate with a remote user across the PSTN 16, e.g., to a standard phone 18 or a mobile or cellular phone 20 through a cellular base station 22, as is understood in the art.
The aforementioned private base station 12 and cordless phones 14A and 14B associated therewith have typically constituted a stand-alone consumer product which, while the phones 14 remain within the short transmission range of such systems 10, behaves like a regular telephone, e.g., phone 18, through the rf link with the private base station 12. The cordless phones 14A and 14B, however, could not also function as a cellular phone, e.g., mobile phone 20, when outside the range of the system 10. Recently, however, cordless phone technology has become more versatile, particularly with private base stations 12 providing cordless services to conventional cellular phones, such as mobile phone 20, also. To accomplish this feat, these digital cordless phone systems utilize an air interface that is in large extent compatible with a standard digital cellular air interface, e.g., the European Groupe Speciale Mobile (GSM) or the Digital Advanced Mobile Phone System (D-AMPS) communications standards.
With reference now to FIG. 2, there is shown a private telephone system, designated generally by the reference numeral 24, used in the system and method of the present invention, which includes a private or home base station (PBS) 26 and a multiplicity of cellular or cellular-compatible phones or terminals, e.g., terminals 28A and 28B, which function as cordless phones when within the proximity of the private base station 26 and which preferably also have an air interface compatible with one of the aforementioned standards, e.g., GSM. Accordingly, the system 24 configuration enables the conventional cellular phones, e.g., terminals 28A and 28B, to connect with the home base station 26 without the need for hardware modifications to the terminals 28A and 28B. Cellular and cordless functionality are instead implemented via software control.
An example of such a digital cordless air interface based on a digital cellular air interface has been described in a recent patent application of the assignee, U.S. patent applicant Ser. No. 08/704,901, of which the present inventor is the inventor thereof, entitled "Method and Apparatus for Adapting Non-Cellular Private Radio Systems to be Compatible with Cellular Mobile Phones," filed Aug. 30, 1996. In this way, an ordinary cellular phone, e.g., terminals 28A or 28B in FIG. 2, can be used either in the cellular mode or in the cordless mode when within the range of the private base station 26, thereby obviating the need for a purely cordless terminal such as the terminals or phones 14A and 14B in FIG. 1.
An obvious advantage of this arrangement is that the private telephone system 24 can reuse most of the hardware utilized in the cellular terminals 28A and 28B. In particular, the same baseband and Intermediate Frequency (IF) radio processing circuits can be reused, as is understood in the art. Reuse of terminal hardware within the private telephone system 24 is also very attractive from a cost point of view since the private base station 26 benefits from the volume production of today's cellular terminals.
Shown in FIG. 3 is a high-level block diagram of various transceiver components (generally designated by the reference numeral 29) within a conventional mobile terminal such as terminals 28A or 28B. As is well understood in the art, four primary transceiving component blocks may be identified therein: a radio block 90, a baseband logic block 92, a control logic block 94 and an audio interface block 96. Within radio block 90, the receive and transmit information is converted from and to rf frequencies, and filtering using baseband or IF circuitry is applied, as is understood in the art. In the baseband logic block 92, basic signal processing occurs, e.g., synchronization, channel coding, decoding and burst formatting, as is understood in the art. Audio interface block 96 handles voice as well as Analog-to-Digital (A/D) and D/A processing. Control logic block 94, via microprocessor control (not shown), coordinates the aforedescribed blocks 90, 92 and 96 and also plays an important role in the Man-Machine Interface (MMI). The functionality of the aforedescribed transceiving blocks will be described in more detail hereinafter, in particular in connection with FIG. 10 and the associated text.
Shown in FIG. 4 is a similar high-level block diagram of transceiving components (generally designated by the reference numeral 30) within the private base station 26. As with the transceiver 29 components of mobile terminal 28 in FIG. 3, four primary transceiving components of the private base station 26 are illustrated in FIG. 4: a PBS radio block 100, a PBS baseband logic block 102, a PBS control logic block 104 and a wireline interface block 106. The PBS radio block 100 is similar to the radio block 90 within the mobile terminal 28, the difference being that the transmission frequency in the terminal 28 must be used for reception in the PBS 26, and vice versa. The IF and DC processing are identical. It should be understood that the baseband logic blocks 92 and 102 in FIGS. 3 and 4, respectively, may be identical. Wireline interface block 106 provides the conversion between standard PSTN 16 or Integrated Service Digital Network (ISDN) signals and the signals for transmission over the air interface. Lastly, PBS control logic block 104, also via microprocessor control, schedules the various processes regarding blocks 100, 102 and 106. It should be understood that, since the respective control logic blocks 94 and 104 constitute microprocessor control, the only modification from standard equipment required is reconfiguration of the aforementioned microprocessors, which does not typically entail any hardware changes.
In accordance with the above description, a simple private base station 26 for cordless communication can readily be implemented with mobile station-based hardware. Thus configured, the private base station 26, through transceiver 30, would support one traffic channel to connect to a single phone, e.g., terminal 28A or 28B. Despite the increased communications functionality, however, this configuration is unable to readily support more advanced telephone features like intercom or multiple voice channels. In an intercom system, for example, one terminal, e.g., cellular terminal 28A, communicates with another terminal within the system 24, e.g., cellular terminal 28B, also connected to the private base station 26. However, in the configuration shown in FIG. 2, since terminals 28A and 28B cannot communicate directly with one another (the terminals cannot hear each other because they both transmit in the TX band, in which they cannot receive, and both receive in the RX band, in which they cannot transmit), intercom functionality may only be implemented indirectly, i.e., by using the private base station 26 as a relay unit. In this manner, the private base station 26, which has an rf link to both terminals 28A and 28B, relays information back and forth between the two terminals.
Current terminal technology, however, supports only one full rate channel on each terminal, i.e., an uplink slot and a downlink slot in Time Division Multiple Access (TDMA) technology, such as applied in GSM and D-AMPS, as is understood in the art. Accordingly, using current terminal technology with only a single radio transceiver to implement the private base station 26, only one channel is allowed in a private telephone system, such as the system 24 configuration shown in FIG. 2, i.e., the private base station 26 therein can only communicate across a single channel regardless of the number of users, i.e., terminals, in the system 24. Thus, intercom and simultaneous multiple voice channels are currently not possible when utilizing (or reusing) conventional terminal technology in a private base station, such as the private base station 26 in FIG. 2.
It would, however, be possible to implement the aforementioned intercom and multiple voice channel functionality in the telephone system 24 of FIG. 2 if the private base station 26 is based on (advanced) terminal technology that can handle multiple channels (multiple time slots and/or multiple carrier frequencies). However, such terminals, at present, do not exist. Additionally, basic multi-slot channels as currently being envisioned for cellular terminals typically require that all timeslots constituting a channel use the same carrier frequency therein. The reason for this frequency restriction is that synthesizers, described further hereinafter, utilized in the terminals and used for the upconversion and downconverssion of signals are not agile enough to switch frequencies from one time slot to the next time slot, necessitating the frequency limitation. This restriction, however, has been solved by using the same carrier frequency on consecutive time slots, thereby facilitating the implementation of a multi-slot channel in a cellular terminal such as terminal 28A or 28B.
Even with this advancement, however, the utilization of such multi-channel technology in a private telephone or cordless system, e.g., system 24 described herein in connection with FIG. 2, for intercom and multiple channel usages limits the performance of the private telephone system. One such performance limitation is due to interference. For example, the aforedescribed private telephone systems 24, which typically operate within a broader overlaying cellular system, may share frequencies therewith because there is no coordination of frequency allocation between the disparate systems. Accordingly, frequency interference between the private "cordless" system 24 and the overlaying cellular system, as well as interference between overlapping private cordless systems 24, is present and has to be prevented, e.g., by adaptive channel allocation techniques applied autonomously in each private cordless system 24.
Such an adaptive channel allocation scheme has been described in a recent patent application of the assignee, U.S. patent application Ser. No. 08/704,846 of which the present inventor is the inventor thereof, entitled "Method and System for Autonomously Allocating a Cellular Communications Channel for Communication Between a Cellular Terminal and a Telephone Base Station," filed Aug. 28, 1996. As discussed in said co-pending application, when establishing a TDMA link, the private system 24 should select a time slot on a carrier frequency which is not already occupied by an overlaying cellular system or overlapping another private cordless system 24. As will be understood to those skilled in the art, applying the multi-slot technology for multiple channels will severely restrict the adaptive channel allocation selection algorithm since the multi-slot concept requires adjacent time slots to be on the same carrier frequency. Therefore, the aforementioned adaptive channel allocation algorithm has to find a carrier with a non-occupied time interval sufficiently large to accommodate the time slots required in the considered private system 24.
Accordingly, there is a need for a private telephone system allowing intercom and multiple voice channel communication capabilities with minimal interference to and from other systems.
It is, accordingly, an object of the present invention to provide intercom and multiple voice channel capabilities within a private telephone system.
It is also an object of the present invention to provide such capabilities with a minimal amount of interference from an overlaying cellular system or any other private telephone systems nearby.
It is a further object of the present invention that the private telephone system utilizes a private base station with a single radio transceiver, both the station and the transceiver being based upon existing terminal technology.